Tire testing machines are used to measure forces generated between a tire and a testing surface, which often is a rotating wheel or drum, to evaluate tire performance, failure and endurance. While these machines are often used to evaluate uniformity of the tire, they may also be used to simulate road conditions and test reaction forces at the tire. For example, tire testing machines have been developed to measure lateral forces on the tire generated by steering and camber of the tire. These machines are useful in evaluating steering, camber, belt edge separation, and slip angle characteristics of the tire, among others.
Existing devices that measure lateral forces require complex calibration making it difficult to obtain reliable results in a consistent fashion. In particular, one existing design includes a tire mounted on a spindle that is rotatable within a bearing. Lubricating pockets are provided all along the length of the bearing to maintain the lubricating film. A load cell is provided around the spindle and measure variations in the loads created within the film. Changes in the film thickness resulting from operation of the testing machine and friction within the bearing causes errors in the forces read by the load cell. The presence of the lubrication pockets inherently causes changes in the film thickness along the length of the bearing and, thus, complex equations have been developed to eliminate these errors and attempt to ascertain a true force from the load cells. While meaningful measurements can be taken with this machine, zeroing of the machine to eliminate the aforementioned errors is time consuming and complicated.
Another known system, referred to as a “piezo-quartz” system, uses piezo-electric gauges to measure loads. In this design the piezo gauges are located around the spindle hub. Since the piezo-electric response requires periodic release of force on the cell to allow it to recharge, the system is not practical for long term force measurements. Also, for a given load, since the system's signal deteriorates with time. As a result of this drift, the piezo quartz system lacks the precision necessary for many testing applications. Consequently, there is a need for a simpler, more reliable lateral force tire testing machine.